During the manufacture of electrical cables, a conductor such as a metallic wire or a plurality of wires may be introduced into an extrusion machine on a continuous basis. The extrusion machine is such that it applies a hot insulating jacket around the circumference of the wires, the insulation material being plastic, rubber and the like.
Following the application of the hot plastic insulation sheath around the wire, the resulting cable is introduced into a cooling bath or trough in order to cool the sheath or jacket and the product is then taken up on a reel or drum.
The resulting finished cable is one which is made up of a single or multicore of electrically conducting wires such as copper, aluminium and the like, totally surrounded by a jacket of insulating material.
The purpose of this cable construction is to provide a signal path such as for an electric signal, along the cable core with the jacket acting as an insulating barrier against any short circuiting of the cable to another cable.
It is accepted that for various applications, a variety of thicknesses of jacketing is specified in order to fulfil certain electrical specifications.
For example, if the cable is to be used for high voltage applications the insulating jacket would be of greater thickness than that applied to a cable for low voltage applications.
Therefore, the amount of thickness of the jacket as well as the material of the jacket are contributory factors to the electrical characteristic requirements and breakdown voltage resolutions that are taken into consideration when a particular cable is manufactured.
In addition to the above considerations there is a further very important consideration that must be taken into account when manufacturing a specific type of cable, namely that the thickness of insulation around the circumference of the cable core should be reasonably constant.
The minimum wall thickness will be required to fulfill the electrical considerations mentioned above, but also it is important that although the minimum jacket thickness is applied a commercial consideration arises, so that excessive jacketing material must not be used which would produce an expensive and non-commercially viable product.
Therefore, there is a requirement and need to ensure that the jacket insulation around the cable core is of minimum thickness but also evenly distributed around the cable core. Due to this requirement, means of measuring the jacket insulation around the core are needed to ensure adherence to the above specifications.